In populations at risk of ischemic vascular events, there is a medical need for chronic long term therapy that is more efficacious than aspirin and other antiplatelet agents such as ticlopidine. Oral glycoprotein (GP) IIb/IIIa antagonists offer the potential of a greater efficacy than currently used antiplatelet drugs. These drugs are likely to have additional therapeutic utility in chronic thrombotic indications, in addition to those presently investigated w/the parenteral compounds. It is not yet known to which extent GPIIb/IIIa receptors have to be blocked to achieve sufficient antithrombotic effects and to which extent the risk of bleeding increased w/80-90% receptor blockade. Individuals who are heterozygous for Glanzmann's thrombasthenia are an interesting model, since these individuals have 50-60% normal number of GPIIB/IIIa receptors and do not bleed. The effects of various regimens of the orally available GPIIb/IIIa antagonist RPR 109891 in patients w/acute coronary syndromes. This study will assess the effects of RPR 109891 on the pharmacodynamic parameters of inhibition of platelet aggregation and bleeding time, assess the pharmacokinetics of this agent, and assess the clinical and laboratory safety of this compound.